A Bidirectional Single Stage DC-DC Converter With High Frequency Isolation Demercil S. Oliveira Jr., Luiz H. S. C. Barreto, Paulo P. Praça, Hermínio M. Oliveira, Matheus Leite and Abrahão A. M. Gomes Energy and Control Processing Group - GPEC Federal University of Ceará Fortaleza-CE, Brazil demercil@dee.ufc.br Abstract— This paper proposes a single-phase bidirectional dc- dc converter feasible to dc distributed power systems and electrical vehicles. The topology provides high frequency galvanic isolation and is able to protect the system during eventual short-circuit in the output side. The current control in the primary side is performed by the duty cycle variation of the primary bridge. It is possible to regulate the dc bus voltage even during voltage dips and short-circuits. The secondary side bridge is driven with constant duty cycle and the power flow is controlled by the variation of the phase shift angle between the two bridges. The basic equations are shown and experimental results are presented and discussed. I. INTRODUCTION With the expansion of the Distributed Generation (DG) reinforced by renewable energy sources in the last decades, and also the increased use of residential and industrial electronic loads, dc distribution systems have been now seriously considered as a promising solution [1] [2] [3]. Since renewable energy systems based on photovoltaic panels, wind turbines, and fuel cells employ dc voltages in a given power conversion stage and considering that local storage may exist, bidirectional dc-dc converters are a must. In order to make distributed power systems feasible, power electronic converters must be fault tolerant. During a short- circuit in a specific load, the main power supply must remain connected during a time interval long enough to drive the local protection device, which will isolate the fault in the circuit without interrupting the supply to the remaining loads. Most of the proposed bidirectional dc-dc converters have asymmetrical circuit configurations that couple two dc links [4]–[8], increasing the complexity and scalability of the structure. In [9], Divan proposed the use of two active bridges in order to achieve high frequency isolation and bidirectional capability. Since then, many works have been focused on the detailed analysis of this converter. In [10], Akagi chose it as the core circuit for the next generation of medium voltage power conversion systems. This paper proposes a new configuration with input current source characteristic while maintaining the characteristics of the dual active bridge proposed in [9]. Also, the proposed concept is extended to ac-dc and ac-ac configurations. The employment of the three-state switching cell concept allows good distribution of losses among the semiconductors and reduction of high frequency harmonic content for both voltages and currents. Thus, it leads to the decrease of rms current levels through the output capacitors, increasing system reliability due to lower operating temperatures. The volume of magnetics is also reduced, with consequent reduction of the very dimensions and losses of the converter. The use of the three-state switching cell also implies reduced switching and conduction losses due to the power shared among the various components. The disadvantages are the increase in the number of semiconductors in the prototype, necessary to the design and implementation of gate drivers, with consequent practical difficulties in the system layout [11]. II. PROPOSED TOPOLOGY A. Topology’s conception Fig. 1a shows the arrangement based on a bidirectional version of the three-state switching cell [11], where a secondary winding can be coupled with transformer T1. In order to avoid transformer saturation, the legs are phase- shifted by 180° and the individual currents must be monitored. Fig. 1b shows an alternative configuration, where the secondary side can be coupled with the transformer T2 and saturation can be avoided by using a series capacitor. Another similar configuration can be obtained using the interleaving technique. The secondary windings may be coupled to inductors L1 and L2, as shown in Fig. 1c, or use a separate core, according to Fig. 1d. Fig. 2 presents the proposed topology, which is obtained from the combination of the dual active bridge converter with the bidirectional three-state switching cell, using the half- Sponsored by the Brazilian Council of Research and Development – CNPQ. 978-1-4673-4355-8/13/$31.00 ©2013 IEEE 2990